Summary of Work: This project concerns the molecular mechanism of hereditary hearing impairment with special emphasis on Waardenburg syndrome (WS). We previously cloned a transcription factor gene, MITF (microphthalmia-associated transcription factor), and found that haploinsufficiency of the gene product is a cause of WS type 2. We also found that MITF is involved in the differentiation of melanocytes, which is essential for normal hearing function as component cells of the cochlea. In the past year we characterized the gene product and the promoter of the MITF gene. MITF appears to be phosphorylated by MAP kinase and other kinases. A putative phosphorylation site is postulated to regulate the function of MITF. We are now examining whether or not this site is phosphorylated and what role it may play in the function of MITF. Approximately 3 kb upstream of the open reading frame of the MITF gene was cloned. This region showed promoter activity in melanoma cells which express PAX3, a gene where mutations are know to cause WS type 1, but not in cells which do not express PAX3. Thus, we hypothesized that PAX3 transactivates the MITF gene. We are now testing this hypothesis using pormoter-reporter gene approaches and electrophoretic mobility shift assays. We are interested in the phenotype caused by a mutation of the mouse Mitf gene. Mice with this mutation (mi-vitiligo) lose hair pigmentation rapidly with aging, suggesting that the mutation may induce apoptosis of melanocytes. We are now testing this hypothesis in cells transfected with MITF bearing the corresponding mutation. In addition, we cloned and characterized genes for Rab proteins, small GTP-binding proteins, which are expressed in melanocytes and other tissues. Our effort to isolate the genes encoding proteins associated with MITF by the yeast two hybrid method is continuing.